Global Navigation Satellite Systems (GNSS) available today or in the near future include GPS, GLONASS, BeiDou Satellite Navigation System (BDS, formerly known as Compass) and Galileo. Multiband GNSS receivers that support multiple systems offer a performance advantage due to the increased number of satellites in view. Conventionally, this is done by providing parallel receiver paths for the different bands, which requires more power and hardware. A more efficient way to architect a multiband GNSS receiver, although not typically done today, would be to have a single down-conversion with a large IF bandwidth that includes all the bands of interest, as shown in FIGS. 1A and 1B. For example, FIG. 1B shows bands B1, B2, B3, and B4 representing four respective frequency bands from different GNSS systems (e.g. GPS, GLONASS, BDS and Galileo) after downconversion to IF by mixer 102. These are all preserved by a wide passband having bandwidth 2*fc established by low pass filters 104-A and 104-B. The individual bands B1, B2, B3 and B4 are then separated by high-speed filtering and processing in the digital domain, which is very efficient in current digital CMOS technology.
One problem with a large IF bandwidth is that it leaves the receiver vulnerable to interference close to the GNSS bands, such as that caused by jammers, due to the limited rejection of a wideband IF filter. There are several existing approaches to making RF receivers more resistant to in-band or close-in RF interference.
Narrower and higher rejection front-end filters can be used in front of the LNA to reject or attenuate the jammer. However, this would only work for a single frequency band receiver. Furthermore, higher rejection and narrower band filters would generally have higher cost and higher insertion loss. Higher insertion loss limits the receiver's sensitivity.
Double-conversion RF receivers for one or more frequency bands can be used to separate the IF signal paths of each frequency band. Each frequency band then has its own IF filter that can be tuned to reject the jammer. This approach increases chip area and power consumption due to duplication of the IF filters, VGA and ADC functions for each frequency band. The LO's used for the second down-conversion mixers are also a source of spurious self-interference in the receiver as their harmonics can mix with the reference frequency and couple to the receiver's RF input.
A high dynamic range ADC can be used to reduce the total receiver gain and thus increase headroom for the jammer signal. This increases the ADC power consumption and sampling frequency. For wide-band IF signals the demands on the ADC get increasingly higher, which puts a limit on how much the dynamic range of the ADC can be increased. For a multiband receiver with a limited power budget, this alone is not sufficient to provide the required jamming immunity.
Increasing current in the IF filters and amplifier can increase the linearity and compression point of these stages, thus enabling the receiver to withstand a higher jammer level before compression and performance degradation. However, this is only an incremental improvement and signal headroom will ultimately be limited by the supply voltage rails. This method also costs additional current. Adjustable biasing has been proposed to minimize power when linearity is not required. See, e.g., Yoshizawa et al, A channel-select filter with agile blocker detection and adaptive power dissipation, IEEE J. Solid-State Cir, Vol 43, No 5, May 2007.
Accordingly, a need remains for efficient methods and apparatuses for multiband receivers with effective and flexible interference rejection, such as that caused by in-band or close-in jammers.